For a recording method by an ink jet printer, which is one typical method among a variety of color recording methods, a variety of ink discharge systems have been developed. Such systems execute recording by generating ink droplets, and attaching the same to any of a variety of record-receiving materials (e.g., paper, film, and fabric, etc.). According to this method, a recording head is not brought into direct contact with the record-receiving material; therefore, generation of noise can be avoided thus achieving silent recording. In addition, due to having the feature that reduced size, increased speed and coloring can be readily achieved, prevalence in recent years has been in rapid progress, and great advancement hereafter is expected.
Inks containing a water soluble dye dissolved in an aqueous medium have been frequently used as conventional inks for fountain pens, felt pens etc., and inks for ink jet recording. Furthermore, to these inks is generally added a water soluble organic solvent in order to prevent pen tips or ink discharge nozzles from clogging with the ink. For these inks, demanded are abilities to generate a recorded image with satisfactory density, probability of avoiding occurrence of clogging at the pen tips and nozzles, favorable drying characteristics on the record-receiving materials, suppression of bleeding, superior storage stability, and the like. Additionally, a variety of fastness such as water resistance, moisture resistance, light resistance and gas resistance has been required of the recorded image.
In the meantime, for recording image or character information on a color display of computers in full color by an ink jet printer, subtractive color mixing generally with three inks having different colors, yellow (Y), magenta (M) and cyan (C), or four different colors including these three colors plus black (K) has been employed, whereby the recorded image is presented in full color. In order to reproduce an additive color mixing image formed with red (R), green (G), blue (B) on a CRT (cathode ray tube) display and the like as strictly as possible using subtractive color mixing, it is desired that Y, M and C, among the coloring matters used in inks, have a hue approximate to the standard and are brilliant, respectively. In addition, long term storage stability, as well as high density of the recorded image, and superior fastness such as water resistance, moisture resistance, light resistance and gas resistance are also required for the inks.
Herein, gas resistance means resistance to a phenomenon of causing discoloration of a printed image via a reaction of an oxidizing gas present in the air and having an oxidizing action such as nitric oxide gas and ozone gas with a coloring matter (dye) of the recorded image on or in recording paper. Particularly, ozone gas is considered as a main causative substance that promotes the fading phenomenon of ink jet recorded images. Since this discoloration phenomenon is characteristic in ink jet images, improvement of the gas resistance, particularly ozone gas resistance, is a significant technical problem in the art.
In recent years, an ink receiving layer may be provided on the surface of an exclusive ink jet paper, which is one of recording papers, in order to obtain a photo image quality according to ink jet recording methods. Although a porous white inorganic substance is often used in this ink receiving layer for accelerating drying of the ink, and reducing bleeding in high quality images, discoloration due to ozone gas is markedly found on such recording papers, in particular.
In order to improve reliability in ink jet recording methods, and to achieve high fastness properties and brightness of the resulting printed image, a variety of ink sets have been proposed as combinations of fundamental three primary colors for use in subtractive color mixing. In particular, according to technical approaches for improvement of fastness properties and brightness of printed images, methods in which the dye is improved have been predominantly employed, and as in, for example, the invention disclosed in Patent Document 1 to Patent Document 3, various novel dyes for yellow, magenta and cyan, respectively, as fundamental three primary colors, or three-color ink sets and the like have been proposed.    Patent Document 1: US Patent Application, Publication No. 2008/0022467    Patent Document 2: PCT International Publication No. 2004/087815    Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2005-105135